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The bridge is a three-span cantileveredtruss bridge at 1,525 ft (465 m) in total length.[3] The center span is longest at 800 ft (244 m) and the maximum truss height is 115 ft (35 m). There are 36 approach spans to the North and 32 to the South. The roadway is seven lanes wide between the shortest (439 ft; 134 m) span and the center to accommodate a toll plaza on the Southbound deck only. The Northbound toll plaza was closed in the 1980s.

The bridge was originally operated by the Massachusetts Port Authority, which, according to the original plan, was to remove the tolls once the $27 million in bonds used to finance the bridge's construction were retired in 1978.[citation needed] Instead, the tolls were increased to 25 cents to cover the closing of the Northbound toll plaza in the 1980s. Starting in the early 1990s the tolls increased sharply to help pay for the Big Dig.[citation needed] As of 2008, the toll is $3.00 for non-commercial cars ($2.50 with an E-ZPass issued by any toll agency, and $0.30 for registered residents of Charlestown and Chelsea with an E-ZPass).

In 1967, the Mystic River Bridge was renamed in honor of Maurice J. Tobin, former Boston mayor and Massachusetts governor. During his term in office (1945–1947), Tobin created Massport and ordered the construction of the Mystic River Bridge. Tobin went on to serve as Secretary of Labor under President Harry Truman before he died in 1953.

In 1973, a gravel truck traveling over the lower deck crashed into a support, collapsing the upper deck onto the truck and killing the driver. Later that year, the bridge reopened after more than two months of repair.[4]

On the morning of July 21, 2014, the bridge's tollbooths were closed and eventually removed for an all-electronic and cashless tolling system, and from that point on all toll charges are paid for via either E-ZPass at the current rate, or "pay-by-mail" where an invoice will be sent to motorist's home via license plate number recognition at the former cash toll rate. This is part of a test by MassDOT to eventually go to a full automatic tolling system throughout the Commonwealth.[7]